The problems with which the inventors were confronted were in the optics field, although the invention is not limited to this field of application.
Certain optical systems are intended to operate over a wide temperature range (for example from about −60° C. to about +100° C.). In these systems, it appears to be necessary to compensate for the displacements, contractions and/or expansions of the focusing optics and also for their index variations, even though minute, due to the thermal variations so as to maintain performance constancy of the system.
To give an example, we mention optical systems intended for infrared imaging, in which certain materials chosen for the focusing optics (generally germanium) prove to be quite sensitive to thermal variations.
It is known to compensate for the displacements, expansions or contractions of the focusing optics by means of active systems (comprising motors controlled by a computing unit with feedback) or passive systems (for example comprising bimetal systems, as proposed in U.S. Pat. No. 4,861,137 or else systems employing materials having a high expansion coefficient, as proposed in U.S. Pat. No. 6,292,312)
Active systems have the major drawback of requiring a current supply and the installation of a feedback loop allowing a computing unit to correct, by means of the motors, the dimensional or positional variations to which the focusing optics are subjected.
As regards the systems of the type proposed in document U.S. Pat. No. 4,861,137, these have the drawback of being limited in terms of deflection. Furthermore, in the particular case of the mounting device described in the aforementioned document, the problem of maintaining parallelism between the axis of the focusing optic, the compensation device for which provides the displacement, and the optical axis of the system in its entirety may arise. Passive systems, more particularly those employing materials having a high expansion coefficient, have the drawback of having a narrow linear deformation range relative to the required use range.